A number of security systems have control panels which report to a monitoring station by telephone in the event that an alarm condition is sensed at the premise. The control panel typically is placed on the telephone line of the premise in front of the connection to the other telephone devices. The control panel can seize the line, when required, and place an outgoing call to the monitoring station.
Other security systems have provided a dedicated telephone line to the control panel and this obviously improves the security of the system, as the telephone line is always available to allow communication between the monitoring station and the control panel. Signals can be sent on the telephone line indicating that the control panel is functioning properly, and in the event that the control panel does not report to the monitoring station, an alarm condition or event can be determined by the monitoring station. The use of a dedicated line provides a much higher degree of reliability, however, there is a substantial increase in costs.
Other arrangements have been proposed for monitoring of the integrity of the communication to individual control panels. One such system is disclosed in U.S. Pat. No. 4,442,320. This system involves the use of a scanning device provided at the switching station of the Public Switch Telephone Network (PSTN) to which the individual control panel is connected. According to this system, a scanner is provided at the switching station and a separate subscriber terminal unit is provided at each premise. Reporting between the scanner and the subscriber terminal unit is completed directly over the telephone line to the switching station. Basically, the connection between the subscriber terminal unit and the scanner is a closed loop and it does not use the standard telephone protocol. The integrity of the telephone connection between the subscriber terminal unit and the scanner at the switching station is assured, as the scanner can send an interrogation signal at a low frequency to the control panel and the control panel can respond. The low frequency signal is typically outside of the audible range, and therefore, these signals can be transmitted even if the telephone is in normal use. In addition, this type of system is based on the scanner forming a connection with the subscriber terminal unit by sending a polling signal and is not able to independently communicate with equipment other than the scanner.
Unfortunately, the telephone lines between individual premises 4 and the switching station of a publicly switched telephone network can vary and there are often parts of the system which have been added which are unable to carry the low frequency signal. Therefore, most of these connections are limited to carrying a signal in the 300 to 3,000 Hz frequency range.